Field of Invention
The present invention belongs to a field of micro/nano fabrication, and particularly relates to a method for fabricating a metallic micro/nanostructure at an optical fiber end-facet by the glue-and-strip method.
Description of Related Arts
By integrating micro/nano devices at the end-facets of optical fibers, exciting and detecting the micro/nano devices by light-waves using optical fiber guided wave technologies, optical functional devices that are simple, flexible and portable can be obtained; meanwhile, since the optical fiber is very fine, such devices can be inserted into very small spaces or in vivo environments. However, using current mainstream micro/nano patterning technologies, such as UV photolithography, electron-beam lithography, focused ion beam etching, etc., it is difficult to fabricate micro/nano patterns and devices on the optical fiber end-facets straightforwardly and efficiently. The reason is that, if UV photolithography or electron-beam lithography is used to fabricate a micro/nano pattern on the optical fiber end-facet, uniform and controlled application of photoresist onto the sample is required. In order to achieve a high fabrication accuracy, the whole optical fiber end-facet needs to have a very uniform photoresist in thickness. However the optical fiber end-facet has a very small size (e.g., the optical fiber used for optical fiber communication generally has a cladding diameter merely about 125 microns), on which the photoresist can't be applied by using a conventional spin coating method in semiconductor industry. A technology in which the optical fiber end-facet is dipped with a drop of photoresist, then the drop of photoresist is blown to be flat by using an air gun, was proposed. But such method is very inaccurate in the control of the thickness of the photoresist, and the fabrication of micro/nano patterns at the optical fiber end-facet by using such a method has a very low yield (Shengfei Feng, Xinping Zhang, Hao Wang, Mudi Xin, and Zhenzhen Lu, “Fiber coupled waveguide grating structures,” Appl. Phys. Lett. 96, 133101 (2010)). If focused ion beam etching method is used to fabricate a micro/nano pattern on the optical fiber end-facet, although the required patterns can be obtained, the fabrication time of focused ion beam etching is very long, and the fabrication cost is very high; moreover, re-calibration of the focusing parameters of the focused ion beam etching instrument is required prior to the end-facet fabrication of each new optical fiber (A. Dhawan, J. F. Muth, D. N. Leonard, M. D. Gerhold, J. Gleeson, T. Vo-Dinh, and P. E. Russell, “Focused ion beam fabrication of metallic nanostructures on end faces of optical fibers for chemical sensing applications,” J. Vac. Sci. Technol. B 26, 2168 (2008)).
The present invention is to firstly fabricate a metallic micro/nanostructure on another substrate, and then glue and strip the structure to the optical fiber end-facet to fabricate the metallic micro/nanostructure on the optical fiber end-facet. A similar glue-and-strip method (“template stripping”) has been used to fabricate metallic micro/nanostructures with high surface quality on planar substrates with relatively large areas (P. Nagpal, N. C. lindquist, S. H. Oh and D. J. Norris, “Ultrasmooth Patterned Metals for Plasmonics and Metamaterials,” Science 325, 594 (2009)), while the above reported method requires a target substrate with a strong adhesion force to a metal, which is not suitable for fabricating a metallic micro/nanostructure at the optical fiber end-facet. It has become a very attractive subject to fabricate a metallic micro/nanostructure at an optical fiber end-facet, and there are various attempts in literatures, but using the glue-and-strip method to achieve it has not been reported. Hereinafter the glue-and-strip method is described, where its main principle is to take advantage of the weak surface binding force between a noble metal and a substrate of a solid material (e.g., glass, mica, silicon, etc). Firstly, the substrate having a weak surface binding force with a metal is etched with a micro/nano pattern, then, a metal is deposited on the substrate having the micro/nano pattern thereon, after that, the metal layer is stripped and transferred to another planar substrate with the metal surface which was originally bound with the substrate facing upward, to complete the fabrication of the metallic micro/nanostructure on the substrate. When using the substrates, such as mica and silicon, that have extremely high surface smoothness, the metallic micro/nanostructures fabricated by the glue-and-strip method also have a very smooth surface. The existing glue-and-strip method is a large area glue-and-strip, i.e., the whole metallic micro/nanostructure on the substrate is stripped and transferred to another substrate.
Besides, due to the limit of slicing accuracy of the optical fiber, prior to fabrication, the end-facet of the optical fiber itself is not completely perpendicular to the optical fiber, such as being 90±1 degrees, that is, a certain angular offset exists. As a result, the conventional methods for fabricating a micro/nano structure at an optical fiber end-facet are difficult to achieve the precise perpendicularity between the plane of the micro/nano structure and the optical fiber, which may affect the final optical performance of the optical fiber end-facet integrated device to some extent.
In view of the above disadvantages in the prior art, the present invention is based on the idea in the glue-and-strip method that a noble metal is stripped off a weakly bound substrate, and thus demonstrates a new method which enables fabricating a metallic micro/nanostructure at an optical fiber end-facet with high quality, and the process of which is simple, fast, and low cost. Moreover, by a process that sets a predetermined angle of 90° between the optical fiber and the substrate, the present invention can adjust the angle between the plane of the micro/nano structure on the optical fiber end-facet and the optical fiber to a precise 90° after the completion of the fabrication.